Many network operators are currently migrating towards layered CS network architectures. Whereas in conventional second generation networks a single component, the mobile switching center (MSC), handles both call control and connectivity, in layered networks these functionalities have been split. More specifically, call control is handled by MSC servers (MSC-Ss) on the control layer, whereas connectivity is handled by media gateways (MGWs) on the transport layer. This separation of call control and connectivity is also referred to as mobile softswitching (MSS).
The MSC-S is responsible for control signalling and contains service logic for setting up, releasing and monitoring CS connections. The MGW, on the other hand, processes and manages the transport of CS payload traffic (such as voice or data traffic). The MGW also provides interconnections to external networks including public switched telephone networks (PSTNs) and public land mobile networks (PLMNs). Separating the service and control functions from payload transport is advantageous for operators who wish to expand into new geographical areas. All they have to do is to install additional MGWs in the new areas for payload transport. The newly installed MGWs may then be connected to existing MSCs and MSC-Ss.
In parallel to the deployment and operation of layered CS networks, there exist efforts to provide new service delivery platforms for offering enhanced multimedia and other communication services. One of these new service platforms is the Internet protocol (IP) multimedia subsystem (IMS) defined by the 3rd generation partnership project (3GPP). IMS represents a service delivery platform for the provision of IP-based multimedia services within emerging all-IP network environments. IMS relies on the Session Initiation Protocol (SIP) for session control. SIP has a strong peer-to-peer character and introduces so-called user agents (UA) that terminate each communication link.
FIG. 1 shows a typical configuration of a communication scenario 100 including a SIP-enabled user terminal 102 connected via a PS access network 104 to an IMS 106. The IMS 106 may in turn be connected via a network-network interface (NNI) 108 to one or more further networks such as another IMS or a layered CS network (not shown).
The IMS 106 has a layered internal structure including a transport layer (or user plane) 110, a control layer (or IMS core) 112 and an application layer (or service plane) 114. The transport layer 110 comprises a media resource function processor (MRFP) implementing payload-related functions and an IMS-MGW interfacing for example a layered CS network.
The control layer 112 comprises several functionalities for processing SIP signalling (SIP proxies) that are collectively called call/session control function (CSCF). More specifically, the control layer 112 includes an interrogating CSCF (I-CSCF) 116 sitting at the network border and providing a single point of entry (and exit), as well as a serving CSCF (S-CSCF) 118 responsible for handling session registrations and for routing of SIP messages to the application layer 114.
On the application layer 114, there exist one or more service components 120 providing the service logic for call services such as telephony services and multimedia services, including a microprocessor with an associated persistent memory 121 storing instructions for execution by the microprocessor. Associated with the IMS services Additionally, one or more database servers including a home subscriber server (HSS) 122 reside on the application layer 114. The HSS 122 stores a service profile for each subscriber and can thus be regarded as the equivalent to the home location register (HLR) in second and third generation networks.
As shown in FIG. 1, the service component 120 and the HSS 122 communicate via the standardised Sh interface. Communication between the HSS 122 and the control layer 112 takes place via the standardised Cx interface, and a likewise standardised IMS service control (ISC) interface is used for communication between the control layer 112 and the one or more service components 120.
It is currently not clear how the evolution from the currently deployed MSS networks to the IMS network 106 as shown in FIG. 1 or other SIP-based networks will take place. However, is clear that network operators with an installed MSS environment, or with the intention to deploy MSS, will want to ensure a smooth transition from MSS and the CS domain to an all-IP solution. From a migration perspective, and also with regard to a re-use of installed equipment, operators might prefer to use CS services (such as CS telephony services) and CS infrastructure during a co-existence period also for IMS subscribers having PS network access.
It can be assumed that in the migration phase from conventional CS networks to SIP-based PS networks, a high percentage of calls will stretch between the CS domain and the PS domain (e.g. the IMS). This assumption is based on the fact that in the beginning there will be only a few IMS subscribers, whereas the majority of possible destinations is still located in the traditional CS domain, or is served by a softswitch solution.
Accordingly, there is a need for a technique for efficiently interconnecting the CS and PS domains.